The present invention relates to a device for arresting the propagation of a buckle in a double-walled pipe wound onto a reel and more particularly in a rigid pipe used for transporting fluids such as hydrocarbons.
A rigid pipe or tube is laid on the seabed usually from what is called a pipelaying vessel. The laying is called S-laying when the pipe adopts the shape of an S between the pipelaying vessel and the seabed and it is called J-laying when the pipe adopts the shape of a J. In the latter case, a guide ramp is provided on the pipelaying vessel, which ramp may sometimes be partially immersed in the water.
The rigid pipe to be laid is stored on the pipelaying vessel either in pipe sections of a given but relatively short length, the pipe sections being joined together as the laying progresses, or it is wound as a great length on a reel, the pipe then being unreeled from the said reel during the laying operation. These laying operations are described in the API (American Petroleum Institute) document xe2x80x9cRecommended Practice 17 Axe2x80x9d from 1987.
When the pipe has left the vessel and while the said pipe is being laid, it is important that the latter undergoes no plastic deformation in bending, which would result in ovalization of the pipe, the said ovalization causing a xe2x80x9cweak singularityxe2x80x9d which would be conducive to the initiation of a collapse. Moreover, when the pipe is laid on the seabed at great water depths (typically greater than 300 m and possibly up to 2000 m and more), the hydrostatic pressure exerted on the pipe may be sufficient to initiate a buckle which has a tendency to propagate along the pipe, in both directions. Of course, the buckle will form preferentially at a xe2x80x9cweak singularityxe2x80x9d when one exists on the pipe. When the buckle occurs, it is then necessary to replace at least that section or portion of the pipe comprising the buckled or collapsed region.
To prevent the propagation of a local buckle or buckles, it has been proposed to provide the pipe with certain devices or means, called buckle arrestors.
Such buckle arrestors are described in the U.S. Pat. Nos. 2,425,800, 3,747,356, 3,768,269 and 4,364,692.
The process in U.S. Pat. No. 3,747,356 consists in linking a cylinder to a cable, in lodging the cylinder inside a pipe section and then in simultaneously unreeling the pipe and the cable so as to keep the cylinder in the pipe section while the latter is being laid, until the pipe comes into contact with the seabed. The cylinder is then brought back up so as to be lodged in another pipe section to be laid, which is joined to the previous section. Consequently, any buckle likely to occur, when laying the pipe, between the pipelaying vessel and the seabed is immediately arrested and therefore not allowed to propagate along the pipe sections. However, such an arrangement provides no solution to or any effectiveness in arresting buckles likely to be propagated after the pipe has been finally laid on the seabed.
U.S. Pat. No. 3,768,269 proposes to locally increase the stiffness of the pipe by placing, at regular intervals, for example at intervals ranging between 100 m and 500 m, reinforcing collars whose length ranges between 1 m and 2.5 m. Such a solution is valid only for pipes laid in sections since the reinforcing collars can be mounted and fastened in the factory to the pipe sections and then transported by the pipelaying vessel to the laying site. When the pipe is long and wound onto a storage reel, it then becomes virtually impossible to wind the pipe with its reinforcing collars onto a reel since they would result in straight or almost straight portions that cannot be deformed when winding the pipe onto the storage reel. In order to mitigate this difficulty, it is conceivable to mount and fasten the reinforcing collars during the laying operations. However, it would then be necessary to interrupt the laying, at regular intervals, so as to mount and fasten the reinforcing collars.
In order to allow the pipe to be wound onto a reel, U.S. Pat. No. 4,364,692 proposes to wind a rod tightly around the pipe so as to form a certain number of turns which can be welded at their ends to the rod itself and/or to the pipe.
According to another embodiment, the turns may be individual turns, by welding their two ends and regularly spacing them apart along that portion of the pipe to be reinforced. As long as the pipe is a single-walled pipe, the increase in the diameter in the reinforced portions may be acceptable. However, when the pipe is of the double-walled or pipe-in-pipe type, that is to say comprising an inner pipe, or liner pipe, and an outer pipe, or carrier pipe, which is slipped over the inner pipe, the increase in the diameter of the outer pipe is unacceptable when transporting and storing long lengths of double-walled pipes.
The object of the present invention is to provide a device for arresting the propagation of a buckle in a double-walled rigid tube or pipe that can be wound onto a reel intended to be stored on a pipelaying vessel or equivalent system, such as a barge, floating platform, etc.
The subject of the present invention is a device which consists of at least one element having symmetry of revolution and being deformable in a direction transverse to the longitudinal axis, said deformable element being placed in the annular space and having an external face which is in contact with the internal wall of the outer pipe, at least at two points, and an internal face which is spaced away from the inner pipe.
One advantage of the present invention lies in the fact that the double-walled pipe retains its external dimensions without any local increase in its outside diameter, while still being sufficiently flexible to be wound onto a take-up reel.
Another advantage is that it is possible to factory-mount the device in the outer pipe before it is positioned around the inner pipe.
Another advantage lies in the fact that the device according to the invention can be lodged in the annular space without modifying the internal arrangement of the double-walled pipe, which generally includes spacers and thermal insulation members.
According to another characteristic of the present invention, the deformable element consists of a core comprising a plastic sheath onto which is wound, with a short pitch, an interlocked or non-interlocked profiled wire, which may constitute a reinforcement similar to an internal carcass or to a pressure vault or a hoop, such as those used in what is called a xe2x80x9crough borexe2x80x9d or xe2x80x9csmooth borexe2x80x9d flexible pipe depending on whether the innermost member of said flexible pipe consists of an inner polymeric sealing sheath or a metal carcass, the structures of such flexible pipes being well-known to experts and described in the API (American Petroleum Institute) documents 17B or 17J from 1988 or 1997, these being included in the description as references.
One advantage of this feature is that it is unnecessary to manufacture a special core, as this is available at the flexible pipe manufacturers and all that is required is to cut the available cores to the desired length and to place them in the outer pipe at the manufacturing premises. In addition, the plastic of the sheath is carefully selected in order to form a thermal barrier, so as to improve the thermal insulation at the buckle propagation arrestor.